a) Field of the Invention
The invention is directed to a microscope objective, particularly for stereomicroscopes, which has a large pupil diameter and is used with incident brightfield illumination.
b) Description of the Related Art
In telescope-type stereomicroscopes, two separate light channels are guided through one and the same objective in order to achieve a stereoscopic impression. As a result, the dimensions of an objective of this kind, especially with regard to the lens diameter, are significantly larger than those of objectives for conventional microscopes or in Greenough-type stereomicroscopes. In addition, requirements for microscope objectives with regard to the correction of the chromatic aberration, field flattening and prevention of distortion are increasingly demanding.
The objectives are usually used in connection with a focal magnification changers and exchangeable tubes. In order to make it possible to use the microscope at magnifications which allow overview imaging of an object as well as detailed viewing without having to change objectives, objectives are required in which there is more space available for suitable magnification changers in the post-magnification area and which also offer a larger usable aperture in the object space.
The use of fluorescence in stereomicroscopes with incident brightfield illumination places new demands on the design of the objectives. Accordingly, with regard to the devices, it is necessary to separate the beam paths of the excitation of the fluorescence and of the observation in the entrance aperture from the device in the objective. This leads to large apertures. Requirements regarding correction are demanding in objectives of this kind for telescope-type stereomicroscopes. In addition, because of the fluorescence excitation the selection of material is limited with respect to transparency and self-fluorescence of the materials which are employed.
EP 1 369 729 A2 describes an objective for telescope-type stereomicroscopes which comprises three optical lens groups. The first lens group is arranged toward the object side and the third lens group is arranged toward a magnification changer. This objective satisfies certain conditions with resect to the diameter of the entrance pupil of the magnification changer arranged downstream of the objective and of the maximum field angle of the objective at low magnification.
JP 2001-147378 describes an objective which is suitable for use in telescope-type stereomicroscopes. Viewed from the direction of the microscope body, they comprise a first lens group with positive refractive power, a second lens group containing at least one cemented triplet, and a third lens group with positive refractive power. These lens groups can also comprise individual lenses in addition to the cemented components or combinations of individual lenses and cemented components.
JP 2001-221955 describes another objective for telescope-type stereomicroscopes. Viewed from the direction of the microscope body, the objective comprises two lens groups of which a first lens group with positive refractive power contains a biconvex cemented component and a second lens group contains at least two cemented components. In addition to the cemented components, the lens groups can also comprise individual lenses as well as combinations of individual lenses and cemented components.
U.S. Pat. No. 6,271,971 describes an objective for telescope-type stereomicroscopes with the primary aim of optimizing the ratio of mounting space to the objective focal length.